DESCRIPTION

Supplying a value of -1 for either the real or effective user ID forces
the system to leave that ID unchanged.

Unprivileged processes may only set the effective user ID to the real user ID,
the effective user ID, or the saved set-user-ID.

Unprivileged users may only set the real user ID to
the real user ID or the effective user ID.

If the real user ID is set (i.e.,
ruid
is not -1) or the effective user ID is set to a value
not equal to the previous real user ID,
the saved set-user-ID will be set to the new effective user ID.

Completely analogously,
setregid()
sets real and effective group ID's of the calling process,
and all of the above holds with "group" instead of "user".

RETURN VALUE

On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.

Note:
there are cases where
setreuid()
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
setreuid().

ERRORS

EAGAIN

The call would change the caller's real UID (i.e.,
ruid
does not match the caller's real UID),
but there was a temporary failure allocating the
necessary kernel data structures.

EAGAIN

ruid
does not match the caller's real UID and this call would
bring the number of processes belonging to the real user ID
ruid
over the caller's
RLIMIT_NPROC
resource limit.
Since Linux 3.1, this error case no longer occurs
(but robust applications should check for this error);
see the description of
EAGAIN
in
execve(2).

EINVAL

One or more of the target user or group IDs
is not valid in this user namespace.

EPERM

The calling process is not privileged
(on Linux, does not have the necessary capability in its user namespace:
CAP_SETUID
in the case of
setreuid(),
or
CAP_SETGID
in the case of
setregid())
and a change other than (i)
swapping the effective user (group) ID with the real user (group) ID,
or (ii) setting one to the value of the other or (iii) setting the
effective user (group) ID to the value of the
saved set-user-ID (saved set-group-ID) was specified.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.3BSD
(setreuid()
and
setregid()
first appeared in 4.2BSD).

NOTES

POSIX.1 does not specify all of the UID changes that Linux permits
for an unprivileged process.
For
setreuid(),
the effective user ID can be made the same as the
real user ID or the saved set-user-ID,
and it is unspecified whether unprivileged processes may set the
real user ID to the real user ID, the effective user ID, or the
saved set-user-ID.
For
setregid(),
the real group ID can be changed to the value of the saved set-group-ID,
and the effective group ID can be changed to the value of
the real group ID or the saved set-group-ID.
The precise details of what ID changes are permitted vary
across implementations.

POSIX.1 makes no specification about the effect of these calls
on the saved set-user-ID and saved set-group-ID.

The original Linux
setreuid()
and
setregid()
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
setreuid32()
and
setregid32(),
supporting 32-bit IDs.
The glibc
setreuid()
and
setregid()
wrapper functions transparently deal with the variations across kernel versions.

C library/kernel differences

At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process
share the same credentials.
The NPTL threading implementation handles the POSIX requirements by
providing wrapper functions for
the various system calls that change process UIDs and GIDs.
These wrapper functions (including those for
setreuid()
and
setregid())
employ a signal-based technique to ensure
that when one thread changes credentials,
all of the other threads in the process also change their credentials.
For details, see
nptl(7).

SEE ALSO

COLOPHON

This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.